Flue gas pipe and method for treating flue gas

ABSTRACT

The invention relates to a flue gas pipe ( 1 ) of an incineration plant, comprising a reaction zone ( 2 ) for breaking down harmful substances and a heat exchanger ( 3 ) for cooling the flue gas (R) which is mounted downstream and fitted with a dioxin catalytic converter ( 4 ). The dioxin catalytic converter ( 4 ) in the heat exchanger ( 3 ) makes it possible to prevent the renewed formation of dioxins by de novo synthesis when the temperature drops.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage of PCT/EP981/04408 filed Jul. 15,1998 and based upon 97112724.6 filed Jul. 24, 1997 under theInternational Convention.

FIELD OF THE INVENTION

The invention relates to a flue gas pipe with a reaction zone and a heatexchanger arranged downstream thereof, as well as to a method oftreating flue gas.

BACKGROUND OF THE INVENTION

A flue gas pipe for the transport and treatment of flue or exhaust gasesis used in a combustion installation, such as a power plant running onfossil fuel or a garbage incinerator.

In the reaction zone the flue gas is thermally or catalytically treated,in order to break down possible harmful substances. Also in a reactionzone of this type so-called adsorbents are used for the capture andstorage of harmful substances. Such a reaction zone is for instance areactor traversed by the flue gas with a specifically acting catalystarranged therein.

In the downstream heat exchanger the flue gas is cooled before itreaches the environment. It is also known to return the heat obtainedfrom the flue gas back into the combustion process, for instance forheating the combustion air. Such a heat exchanger suitable for thispurpose is also known as regenerative heat exchanger.

In the combustion of organic matter or of household refuse whichrepresents a mixture of organic and synthetic materials, besides carbonmonoxide, nitrogen oxides and hydrocarbons, dioxins can result, and evenin minimal amounts are toxic to humans. The “dioxins” are a collectionof cyclically halogenated aromatic polyethers. To this group pertain thecyclic ethers (furanes), as well as the cyclic diethers (the actualdioxins). As particularly toxic representatives the polychlorinateddibenzodioxin (PCDD) and the polychlorinated dibenzofurane (PCDF) shouldbe mentioned here.

In order to eliminate the mentioned harmful substances, specificallyacting catalysts can be used in the reaction zone of the flue gas pipe.

The EP 0 447 537 B1 describes a catalytic conversion system for theelimination of nitrogen oxides and dioxins from the flue gas of agarbage-incinerating plant. Here a standard DeNOx-catalytic converter,which working according to the known SCR-process of the selectivecatalytic reduction with the addition of a reducing agent, reducesnitrogen oxides to molecular nitrogen and water, and an oxidationcatalytic converter which oxidizes dioxins are arranged in series. Bothcatalytic converters basically contain titanium dioxide TiO₂, tungstentrioxide W0 ₃, vanadic pentoxide V₂O₅ and optionally molybdenum trioxideMoO₃.

Also from DE 43-04 323 A1 a catalytic conversion system for the removalof nitrogen oxides NO_(x), carbon monoxide Co and dioxins is known.There in the flow direction of the flue gas of a combustioninstallation, a reduction or SCR-catalytic converter precedes anoxidation catalytic converter and the latter precedes a dioxin catalyticconverter.

There are also known catalytic converters which, contrary to theSCR-catalytic converters, remove the nitrogen oxides contained in theflue gas without using separate reduction agents. These catalyticconverters are generally defined as SCD-catalytic converters(SCD=Selective Catalytic Decomposition).

It is also known to thermally break down the dioxins contained in theflue gas by heating it in a reaction zone to a temperature over 800° C.

Further from EP 0 502 443 B1 it is known to perform the decomposition ofnitrogen oxides directly in a regenerative heat exchanger, by means of aSCR-catalytic converter arranged therein.

However it has been found that when using a heat exchanger arrangeddownstream of the reaction tone, the flue gas reaching the environmentcontains again dioxins in amounts which can not be dismissed.

OBJECT OF THE INVENTION

Therefore it is the object of the invention to reliably avoid dioxinemissions into the environment in a flue gas pipe with a reaction zoneand a subsequently arranged heat exchanger.

SUMMARY OF THE INVENTION

According to the invention this task is achieved by a flue gas pipe witha reaction zone for breaking down harmful substances contained in theflue gas and a heat exchanger for cooling the flue gas arrangeddownstream thereof, whereby the heat exchanger comprises a dioxincatalytic converter designed for breaking down dioxins.

The invention starts out from the concept that in a flue gas of acombustion installation at a temperature between 250 and 400° C. arenewed formation of dioxins takes place through a so-called de novosynthesis. The mechanism of this de novo synthesis is described indetail in the VDI Reports No. 634, 1987, pages 557 to 584. Dioxins areformed in the presence of oxygen from organic compounds which have notburned and metal chlorides, whereby in a first stage available heavymetal components catalyze a chlorine formation from the mentioned metalchlorides. Since the compounds responsible for a de novo synthesis arepresent in the flue gas of a power plant running on fossil fuel or of agarbage incinerator even after the catalytic treatment of the flue gastakes place, each heat exchanger arranged downstream of the reactionzone, as a result of the performed temperature reduction, represents apotential source for renewed dioxin formation. Particularly since, as arule, in a subsequent beat exchanger the flue gas is cooled to atemperature of less than 400° C.

However such a renewed formation of dioxins in the flue gas after thealready performed treatment of harmful substances is safely avoided whenthe heat exchanger arranged downstream of the reaction zone contains adioxin catalytic converter equipped for the breakdown of dioxins. Inthis way on the one hand the renewed formation of dioxins is avoidedwhen the temperature drops, and on the other hand already formed dioxinsare decomposed into inoffensive substances.

As a dioxin catalyst for instance a catalyst can be used which containsTiO₂, R\WO₃, V₂O₅ and optionally MoO₃. The content of V₂O₅ has therebyto be adjusted to the conditions of the flue gas. A catalyst in plate,honeycomb or pellet form is suitable.

It is particularly cost-effective and technically simple when the heatexchanging element of the heat exchanger is formed by the dioxincatalytic converter itself, and the heat exchange takes place on thesame. For this purpose a correspondingly coated honeycomb orplate-shaped catalyst is particularly suited, which due to the largespecific surface has also a high heat storage capacity.

The efficiency of the combustion installation increases when the usedheat exchanger is a regenerative heat exchanger. Such a regenerativeheat exchanger on one hand extracts the heat front the flue gas and onthe other band returns the extracted heat back into the combustionprocess of the combustion installation. This can take place for instanceby heating the combustion air.

The dioxin content of the flue gas can be further reduced when thereaction zone comprises an additional dioxin catalytic converter or adioxin adsorber. For instance a honeycomb or plate-shaped catalyst of analready mentioned composition on a titanium dioxide basis is suitable. Adioxin catalytic converter in the form of charge or in the form ofpellets is also conceivable.

Particularly the arrangement in series of a dioxin catalytic converterwith a DeNOx-catalytic converter in the reaction zone offers advantagesin that, under substantially identical reaction conditions, it ispossible to remove simultaneously nitrogen oxides and dioxins from theflue gas. Since both catalytic*converters have a similar chemicalcomposition, it is possible to provide a single catalyst carrier forboth catalytic converters.

The invention also comprises a method for flue gas treatment, whereby aflue gas of a combustion installation flows first through a reactionzone for the removal of harmful substances and is subsequently passedover a downstream-arranged heat exchanger for cooling, whereby the fluegas is contacted in the heat exchanger with a first dioxin catalyticconverter breaking down the dioxins.

A particularly effective decomposition of dioxins and nitrogen oxidescan be achieved when the flue gas in contacted in the reaction zonefirst at a temperature of 300 to 500° C. with a DeNOx catalyticconverter and/or with an additional second dioxin catalytic converterand subsequently is passed for cooling over the heat exchanger with thefirst dioxin catalytic converter.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a schematic section through a flue gas pipe with a reactionzone and a heat exchanger arranged downstream thereof; and

FIG. 2 is a schematic section through a flue gas pipe with a reactionzone and a heat exchanger arranged downstream thereof, whereby thereaction zone comprises a combined DeNOx/dioxin catalytic converter.

SPECIFIC DESCRIPTION

FIG. 1 represents schematically a flue gas pipe 1 of a combustioninstallation, particularly a garbage-incinerating plant not illustratedhere in greater detail. The flue gas pipe 1 comprises a reaction zone 2and a heat exchanger 3 arranged downstream thereof, which is designed asa regenerative heat exchanger. For the exhaust gas treatment, the fluegas G, which besides harmful gaseous substances such as nitrogen oxides,carbon monoxide, unburnt organic compounds and dioxins, contains alsosolid components such as dusts containing heavy metals and metalchlorides, passes at first through the reaction zone 2 for the breakdownof harmful substances. There the harmful gaseous substances are removedfor instance thermally or catalytically. Normal temperatures for thecatalytic removal of nitrogen oxides or dioxins range thereby betweenapproximately 300 and 500° C. For a thermal decomposition of the dioxinsthe flue gas is heated above 800° C.

After a successful removal of harmful substances, the flue gas G flowsthrough the subsequent heat exchanger 3. There the flue gas G is cooleddown to a temperature lower than 400° C. The extracted heat is directedagain to the combustion process of the combustion installation. For thispurpose the flue gas G flows through a heat exchanging element of theheat exchanger 3, which in the embodiment example of FIG. 1 is a dioxincatalytic converter 4 having a honeycomb shape. The dioxin catalyticconverter consists basically of titanium oxide TiO₂ and tungstentrioxide WO₃. The freely accessible surface has a content of vanadicpentoxide V₂O₅ of approximately 20%. By rotating the dioxin catalyticconverter 4 about the longitudinal axis of the flue gas pipe 1, theheated part of the dioxin catalytic converter 4 is turned into alaterally arranged fresh air duct, whereby the stored amount of heat isreleased to a clean gas R flowing through the fresh air duct. Thethereby heated clean gas R is fed (here not illustrated) to thecombustion of the combustion installation. After passing through theheat exchanger 3, the flue gas G is finally released into the atmospherethrough a stack which is not illustrated

As a result of the dioxin catalytic converter 4 arranged as heatexchanging element in the heat exchanger 3, a renewed formation ofdioxin through de novo synthesis can be prevented during the temperaturedrop in subsequently arranged heat exchanger 3.

Like FIG. 1, FIG. 2 shows a flue gas pipe 1, wherein a DeNOx catalyticconverter 5 and a dioxin catalytic converter 6 connected in series arearranged in the reaction zone 2. Here the two catalytic converters arecarrier catalysts, each with a honeycomb-shaped carrier. Basically thecarrier consists in both catalytic converters 5, 6 of titanium dioxideTiO₂ and tungsten trioxide WO3. The DeNOx catalytic converter 5 and thedioxin catalytic converter 6 each have vanadic pentoxide V₂O₅ in thesurface freely accessible to the flue gas.

What is claimed is:
 1. A method of treating a flue gas, comprising thesteps of: (a) obtaining from a combustion of organic substances andsynthetic materials a flue gas containing carbon monoxide, nitrogenoxides, hydrocarbons and dioxins; (b) passing the flue gas obtained instep (a) through a flue gas pipe and in a catalytic reaction zonecontaining catalysts at least for removal of the nitrogen oxides,removing harmful components from said flue gas including carbonmonoxide, nitrogen oxides and dioxins, to produce a flue gas at atemperature above 400° C. susceptible to de novo synthesis of dioxins oncooling; (c) then passing the flue gas at said temperature above 400° C.through a previously cooled regenerative heat exchanger constituting adioxin converter and disposed along said pipe downstream of saidreaction zone to cool the flue gas to a temperature below 400° C.without de novo synthesis of dioxin; and (d) releasing the flue gascooled in step (c) into the environment.
 2. The method defined in claim1 wherein, in said reaction zone, the flue gas obtained in step (a) isfirst contacted with a DeNox catalytic converter at a temperature of 300to 500° C. to break down nitrogen oxides and is then contacted in saidreaction zone with a further dioxin catalytic converter.
 3. A flue gaspipe connected to a system for combustion of organic substances andsynthetic materials and through which a flue gas containing carbonmonoxide, nitrogen oxides, hydrocarbons and dioxins is passed, said fluegas pipe having a catalytic reaction zone containing catalysts at leastfor removal of the nitrogen oxides and removing harmful components fromsaid flue gas including carbon monoxides, nitrogen oxides and dioxins toproduce a flue gas at a temperature above 400° C. susceptible to de novosynthesis of dioxins on cooling; and a regenerative heat exchangerdownstream of said reaction zone and constituting a dioxin convertercapable of cooling the flue gas to a temperature below 400° C. withoutde nova synthesis of dioxin.
 4. The flue gas pipe defined in claim 3wherein said reaction zone comprises a dioxin catalytic converterconnected in series with a further dioxin catalytic converter.